Stringent emission regulations on light-duty gasoline vehicles such as US LEV III and EURO 7 demand advanced three-way conversion (TWC) catalyst systems. By 2025, for instance, super ultra-low emission vehicles (SULEV) are projected to have a substantial market share in North America, requiring combined non-methane hydrocarbon (NMHC) and NOx emissions of less than 30 mg/mile under warranty of 15 years and 150K miles on a fleet average.
It is generally recognized that TWC catalyst systems generate ammonia. U.S. Pat. No. 6,109,024 to Kinugasa et al. describes an exhaust gas purification device for an internal combustion engine that locates both an ammonia adsorbing-denitrating catalyst and a NOx adsorbing-reducing catalyst downstream of a TWC catalyst. U.S. Pat. No. 8,661,788 to Qi et al. describes a system that positions an ammonia-SCR catalyst downstream of a TWC catalyst.
For the purpose of reducing tailpipe hydrocarbon (HC) emissions, HC adsorbent components (typically one or more microporous zeolite based materials) have been proposed to delay HC release, in particular, during engine cold start. U.S. Pat. No. 7,163,661 to Yamamoto et al. contemplates addressing release of cold start hydrocarbons (HCs) using an emissions treatment system comprising an HC adsorbing/purifying catalyst downstream of a first three-way catalyst and a second three-way catalyst downstream of the HC adsorbing/purifying catalyst.
There is a need to develop high performance TWC systems meeting the tightening emission standards. In particular, there is an ongoing need to reduce both HC breakthrough during cold start and NOx breakthrough at hot stage.